Shape memory alloy with ductility and a making process of the same

ABSTRACT

A Ti—Ni shape memory alloy with ductility, including Ti of 50˜66 atomic % in a composition, and in which precipitation of TI 2 Ni phases at grain boundaries is suppressed.

FIELD OF THE INVENTION

[0001] The present invention relates to a Ti—Ni shape memory alloy and amaking process of the same. More particularly, the present inventionrelates to a new Ti—Ni shape memory alloy which is excellent inmechanical properties with which the Ti—Ni shape memory alloy can be putto practical use as an actuator for micro machines such as a microvalve. The present invention also relates to a making process of theTi—Ni shape memory alloy.

DESCRIPTION OF THE PRIOR ART

[0002] A Ti—Ni alloy has been known as a shape memory alloy. A makingprocess in which the Ti—Ni shape memory alloy is produced as a thin filmhas also been known. Besides, it has been known that a Ti—Ni alloy withexcessive Ti, the amount of which is 50˜66 atomic % in a composition,has higher R-phase transformation temperature than those of Ti—Ni alloyseither with excessive Ni in a composition or with a composition in whichTi and Ni have an equal atomic ratio.

[0003] While these Ti—Ni alloys with lower transformation temperaturehas been put to practical use, the Ti—Ni alloy with excessive Ti has sopoor mechanical properties and is so brittle that, in fact, it has notbeen used so far.

[0004] The present invention has an object to overcome the defect of theTi—Ni alloys above-mentioned and to provide a new Ti—Ni shape memoryalloy with excessive Ti, which can be actuated at room temperature andhas enough mechanical properties to be put to practical use.

[0005] This and other objects, features and advantages of the presentinvention will become more apparent upon a reading of the followingdetailed specification and drawing, in which

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a photo showing a structure of a Ti—Ni alloy thin filmobtained by annealing at 773K for 5 minutes;

[0007]FIG. 2a is a photo showing a structure of a Ti—Ni alloy thin filmobtained by annealing at 873K for 3 minutes,

[0008]FIG. 2b is a photo showing a structure of a Ti—Ni alloy thin filmobtained by annealing at 873K for an hour,

[0009]FIG. 3 illustrates several stress-strain curves of a Ti—Ni alloythin film with the structure as shown in FIG. 2b;

[0010]FIG. 4 illustrates several stress-strain curves of a Ti—Ni alloythin film with the structure as shown in FIG. 1;

[0011]FIG. 5 illustrates several stress-strain curves of Ti—Ni alloythin films obtained by annealing at 873K for 3˜60 minutes,

[0012]FIG. 6 illustrates a diagram showing a relationship betweenplastic strain of Ti—Ni alloy thin films at room temperature andannealing temperatures;

[0013]FIG. 7 illustrates the result of a heat cycle test relating to theTi—Ni alloy thin film as shown in FIG. 1; and

[0014]FIG. 8 is a photo showing a structure observed in a usual ingotmaterial.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides a Ti—Ni shape memory alloy withductility, which includes Ti of 50˜66 atomic % in a composition and inwhich precipitation of Ti₂Ni phases at grain boundaries is suppressed.More particularly the present invention provides a Ti—Ni shape memoryalloy with ductility, which exhibits plastic strain of not less than 15%at room temperature.

[0016] The present invention further provides a making process of aTi—Ni shape memory alloy with ductility, which includes Ti of 50˜66atomic % in a composition. The process comprises the steps ofcrystallizing an amorphous Ti—Ni alloy at 600˜900K for less than an hourand then cooling down to room temperature slowly in order to suppressprecipitation of Ti₂Ni phases at grain boundaries. More particularly,the present invention provides a making process in which thecrystallization is conducted at 800˜900° C. for less than 10 minutes.

[0017] Suppressing precipitation of Ti₂Ni phases at grain boundariesremarkably improves mechanical properties of a Ti—Ni alloy withexcessive Ti.

[0018] Any special condition is not required for alloy compositionexcept that the amount of Ti is within the range of 50˜65 atomic % Thealloy of the present invention mainly consists of two metallic elements,i.e., Ti (titanium) and Ni (nickel), but as far as both Ti₂Ni and TiNiphases are precipitated in a usual ingot material, other elements thanTi and NI may be added or mixed as an impurity within the range wherecrystalline structures peculiar to those two phases are preserved.

[0019] On the other hand, in the case that the amount of Ti is less than50 atomic %, Ti₂Ni phases are not precipitated in a usual ingotmaterial. In the case of Ti of beyond 66 atomic %, TiNi phases are notprecipitated and, as a result, effects of the present invention are notexhibited.

[0020] Once a usual ingot material is subjected to high temperature,Ti₂Ni phases are preferentially precipitated at grain boundaries whichare stable in energy and this precipitation severely deterioratesmechanical properties of the material. For example, FIG. 8 illustrates astructure observed in a usual ingot material (H. C. Lin, Shyi-kaan Wu,and J. C. Lin: Proc. ICOMAT'92, J. Perkins ed., Monterey Institute ofAdvanced Studies, Monterey, Calif., 1992, pp 875-880). In FIG. 8, anestimated scale is inserted into for reference. Precipitation of Ti₂Niphases at grain boundaries is confirmed.

[0021] In the present invention, in order to prevent Ti atoms fromdispersing to grain boundaries and make an alloy in which precipitationof Ti₂Ni is suppressed, an amorphous alloy is crystallized in atemperature range of 600˜900K for a short time, i.e., less than an hour.

[0022] Crystallization time is varied according to the size and theshape of an alloy, but a short time is preferable. Time suitable forcrystallization is within the middle of minutes of two figures Tenminutes or shorter are considered to be more suitable.

[0023] A typical annealing condition is exemplified as 773K for 5minutes. Preferably, a condition of 800˜900K for less than 10 minutes isexemplified. In the latter condition, plastic strain of not less than15% is obtained at room temperature, this realizing a Ti—Ni shape memoryalloy which is not fractured in working and practical use.

[0024] An amorphous Ti—Ni alloy is produced as a thin film by a vapordeposition or any arbitrary technology. A making manner of it is notrestricted.

[0025] The alloy of the present invention as a thin film is expected tobe applied to an actuator for a micro machine and its importance isemphasized.

[0026] Now, an alloy and its making process of the present inventionwill be described more in detail by way of examples, but it is needlessto mention that the present invention is not restricted to theseexamples.

EXAMPLES

[0027] Using a target of a Ti—Ni alloy, an amorphous thin filmconsisting of a Ti-48.3 atomic % Ni alloy with the thickness of about 7μm was formed on a glass substrate. The alloy thin film was annealedwithin a temperature range of 600˜900K and then cooled down to roomtemperature slowly. A structure of the alloy thin film after annealingwas observed by an electron microscope. FIG. 1 and FIGS. 2a and 2 b arephotos of a typical structure In the alloy thin film subjected toannealing at 773K for 5 minutes, as shown in FIG. 1, a slight amount ofoxides are confirmed at grain boundaries, but Ti₂Ni phases are scarcelyprecipitated. The same stricture as this is confirmed in the alloy thinfilm annealed at 873K for 3 minutes, as shown in FIG. 2a. On thecontrary, in an alloy thin film subjected to annealing at 873K for anhour, as shown in FIG. 2b, precipitate of wedged-shaped Ti₂Ni phases isobserved at grain boundaries, especially at triple points of grainboundaries.

[0028] The stress-strain curves of the Ti—Ni alloy thin film annealed at873K for an hour, as shown in FIG. 3, are the results of measurement atseveral testing temperatures. Even at each temperature, strain relatingto martensitic transformation is confirmed, but the alloy thin film wasfractured during elastic deformation of martensite phases. Plasticdeformation is not confirmed. Plastic strain is zero and no ductilityexhibits at the time of the elastic deformation.

[0029] On the contrary, it is confirmed from several stress-straincurves as shown in FIG. 4 that the Ti—Ni alloy thin film annealed at773K for five minutes exhibits plastic deformation as high as 5˜12%after martensite phases is yielded. It is estimated from this fact thatthe alloy thin film has enough ductility to be put to practical use. Theductility of the alloy thin film is such a property as has never beenobtained in conventional ingot materials.

[0030] The stress-strain curves as shown in FIG. 5 are the results ofmeasurement at room temperature, i.e. 20° C. Specimens were Ti—Ni alloythin films which were annealed at 873K for 3˜60 minutes. Plastic straindeteriorates with the length of annealing time. The Ti—Ni alloy thinfilm annealed at 873K for 3 minutes exhibits plastic strain of about25%, which is the best of all specimens.

[0031] The diagram as shown in FIG. 6 shows a relationship betweenplastic strain of Ti—Ni alloy thin films at room temperature andannealing temperatures. Annealing time of these alloy thin films was 5minutes and was common to all specimens. Plastic strain of not less than15% is obtained at room temperature in the case that the annealingcondition is 800˜900K for less than 10 minutes. The ductility realizes aTi—Ni shape memory alloy which is not fractured in working and practicaluse

[0032] From FIG. 7, which illustrates a shape memory property of thealloy thin film annealed at 773K for 5 minutes, it is confirmed thathigh transformation temperature which is one of the characteristics ofthe alloy with such a composition is maintained and that a completeshape memory effect exhibits at not less than room temperature. It isparticularly important that the shape memory effect as shown in FIG. 7is not resulted from martensitic transformation observed in a usualingot material, but from R phase transformation. Since the strain of Rphase transformation is smaller than that of martensitic transformation,the amount of plastic deformation which is introduced at the time oftransformation is small enough. This fact as well as improvement ofmechanical properties above-mentioned is thought to be useful forimproving reliability of use for a long time. Besides, since temperaturehysteresis is small, an actuator with high response can be possiblyrealized.

What is claimed is:
 1. A Ti—Ni shape memory alloy with ductility,including Ti of 50˜66 atomic % in a composition, and in whichprecipitation of Ti₂Ni phases at grain boundaries is suppressed.
 2. TheTi—Ni shape memory alloy with ductility as claimed in claim 1 , whereinthe Ti—Ni shape memory alloy exhibits plastic strain of not less than15% at room temperature.
 3. The Ti—Ni shape memory alloy with ductilityas claimed in claim 1 , wherein the Ti—Ni shape memory alloy is producedas a thin film.
 4. The Ti—Ni shape memory alloy with ductility asclaimed in claim 2 , wherein the Ti—Ni shape memory alloy is produced asa thin film.
 5. A making process of a Ti—Ni shape memory alloy withductility, including Ti of 50˜66 atomic % in a composition, whichcomprises the steps of crystallizing an amorphous Ti—Ni alloy at600˜900K for less than an hour and then cooling down to room temperatureslowly in order to suppress precipitation of Ti₂Ni phases at grainboundaries.
 6. The making process as claimed in claim 5 , whereincrystallization is conducted at 800˜900K for less than 10 minutes. 7.The making process as claimed in claim 5 , wherein the Ti—Ni shapememory alloy is produced as a thin film.
 8. The making process asclaimed in claim 6 , wherein the Ti—Ni shape memory alloy is produced asa thin film.